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Newbie here. I do a bit of home electrical work at the most basic level. (Replacing outlets and switches. Running a new line off an existing outlet. Nothing that would even elevate me to an amateur status.) I am smart enough to know to always turn off the breaker and to always double-check to make sure it is off. The rest, I know, is enough to make me dangerous.

So, I'm trying to figure out what is going on here. I am in US. I have a 240V 30A breaker that powers two 240V heaters in two different rooms. Both heaters draw less than 8A and both have their own dedicated thermostat. The heaters do not have built-in thermostats. I am replacing the thermostats with 'smart' thermostats. (The Mysa thermostat.) Picture of the existing thermostats below.

Old thermostat / front and back.

I did some research. I watched YouTube videos on 240V. I read the heater wiring diagrams. I read (twice) the Mysa install documentation. I pulled off the first thermostat on that circuit and it was exactly what I expected. 2 wires coming in from the breaker (red and black) and 2 wires going to the heater. I followed the instructions, triple-checked my work, put the new thermostat on the wall, and threw the breaker back on. Ta da! Smart thermostat bliss!

Breaker back off. Removed the other thermostat. WTF???!!! There were only 2 wires in the box. One was connected to a red. The other was connected to a corresponding black. The other red and black were not connected and had wire nuts capping them off.

The heater works, but based on my (VERY LIMITED) understanding of 240V, I have no clue what is going on here. Or even how this is supposed to be working. The heater is definitely a 240V heater. Is there something sneaky / sketchy going on inside the wall that I can't see?

I'd appreciate it if anyone has suggestions.

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    Can you post a photo of the inside of the box in question please?
    – ThreePhaseEel
    Dec 30, 2019 at 23:53
  • When you say "wire nuts capping them off", do you mean together?
    – Harper - Reinstate Monica
    Dec 31, 2019 at 0:42
  • I don't have a picture of the specific box that I'm referring to. I usually take pictures of anything before I start taking it apart, but in this case I did not since I was so confused I just stopped. I stared at it for a while thinking I was hallucinating. :-) There were 3 wires. A ground wire (green) and the two that were connected to one side of the thermostat. Harper - On the two wires that were not connected to anything, they each had a wire nut on them, but they were not connected to each other.
    – Jim McBee
    Dec 31, 2019 at 2:27
  • Ok here's the thing. I gather you haven't gotten this far yet, which is fine but we need to be there now. Wires are grouped into cables. (Or sometimes conduits). The crux of the question is how these wires are grouped into cables. Are the two on the 'stat from the same cable, or different cables?
    – Harper - Reinstate Monica
    Dec 31, 2019 at 15:02

1 Answer 1

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With the limited info provided I will guess that it is a 240v heater as the other one was but this time they only used a single pole thermostat to control the power, it works the negative here is that there will always be a hot conductor in the baseboard unless the breaker is turned off, this used to be quite common, or was used with multiple heaters in 1 room. Each of the switches in the thermostat can control a 240v load, but by not breaking both hot conductors there is always power to the heater but no current flow until the switch closes providing a path for current flow.

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  • That makes sense and is very possible. It is most definitely a 240v heater. Pretty sure only two wires are coming in to the heater enclosure through the conduit. I think that is the way the Mysa is wired. It has 2 black wires (labeled Load and L1) and 1 red wire (labeled L2). They instruct you to connect the L2 both the incoming and outgoing leg. So, somewhere in a junction / wiring box, there is another hot? Next time I'm at this location, I can start pulling off plates and tracing wires. If I want to use the Mysa, I'll have to get both load wires pulled in to this box.
    – Jim McBee
    Dec 31, 2019 at 2:35
  • Mysa wiring diagram. Not sure if I can post this or not. help.getmysa.com/hc/article_attachments/360019134431/…
    – Jim McBee
    Dec 31, 2019 at 2:35
  • It looks like they are only breaking 1 leg but the electronics are 240v so you will need the other hot to power the thermostat. I used to use 4 square boxes and single gang mud rings to do this sometimes the other conductor is in the box back where you may not notice it look there first.
    – Ed Beal
    Dec 31, 2019 at 16:50
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    I wanted to follow up and let you know that you were spot on. When the thermostat was put in, they only ran a single leg up to it. I traced the wires back to a single gang box that had the other leg. I pulled two new wires through the conduit to extend the second leg and wired up the thermostat. It worked like a charm. I greatly appreciate your insight on this. It was very educational. Pushing 10 gauge wire through 8' of flexible conduit was another matter entirely.
    – Jim McBee
    Feb 9, 2020 at 5:58
  • Thanks, glad you got it.
    – Ed Beal
    Feb 9, 2020 at 22:57

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